1. Field of the Invention
The present invention relates to an etching method of silicon materials, particularly to an etching method of silicon materials employed for the preparation of analytical samples for the analysis of impurities contained in silicon materials which are used for a semiconductor.
2. Description of the Related Art
A silicon material composed of Si or SiO.sub.2 crystals has been used as a semiconductor substrate or the like. When impurities such as sodium, potassium and/or iron are contained in these crystals, even in a trace amount, the electrical properties of the semiconductor device fabricated from them become unstable. Thus, impurities are known to be a large influence on the properties of semiconductor device. Accordingly, it is necessary to suppress the content of impurities in the silicon material as low as possible in order to make high the properties of the semiconductor device. However, as a preliminary step prior to the suppression of the impurity content, precise analysis of the impurity concentration in the silicon material is inevitable.
With a tendency to the miniaturization of a semiconductor substrate in recent years, it has come to be clear that the crystallinity in the vicinity of the surface of a silicon wafer is a very important factor for controlling the properties of the substrate. Described specifically, in the vicinity of the surface of the silicon wafer, that is, about 10 .mu.m deep from the surface (so-called Denuded Zone), micro defects such as oxygen precipitate have an influence upon the electrical properties such as withstand voltage or the like; or metal impurities which have entered from the surface of the wafer and mixed therein during the step such as ion implantation are diffused even to the surface again at the time of subsequent thermal treatment, thereby forming precipitation defects. Such problems caused by metal impurities have been serious. Accordingly, for the purpose of studying the properties in the vicinity of the surface, there is a strong demand for a highly-sensitive detection method of metal impurities which method facilitates the measurement of the concentration as low as 1 ppt to 0.1 ppt.
Frameless atomic absorption photometer, inductively coupled plasma mass spectrometer or the like is generally employed for the measurement of the impurities contained in the silicon materials. For the preparation of a sample solution provided for the frameless atomic absorption photometer, the following methods are conventionally adopted. Specifically, in the first method, a sample solution for frameless atomic absorption analysis is prepared by subjecting a silicon material to direct etching with a mixed solution of hydrofluoric acid and nitric acid, evaporating the solution to dryness to obtain a residue and then adding pure water to the residue to dilute into a fixed volume. In the second method, a sample solution for frameless atomic absorption analysis is prepared by heating a mixed solution of hydrofluoric acid and nitric acid at 100.degree. C. to generate acid vapor, etching the silicon material with the acid vapor and then using it as it is.
It is however very difficult to carry out high purification of the reagents used in the above methods for the analysis of the impurities contained in the silicon material. For example, even the reagents purified by non-boiling distillation method or ion exchange method already contain impurities (e.g. Na, K) in an amount not smaller than 10 ppt. If the silicon material is etched using such reagents which have not been subjected to further purification, the sample itself provided for analysis is contaminated.
FIG. 1 is a schematic diagram of an apparatus used upon the etching of a silicon material by dissolving acid vapor in water in which the silicon material has been dipped. This apparatus has a closed top container 1 and in this closed top container 1, installed are a sample container 3 which contains therein pure water W and a silicon material sample M, a hydrofluoric acid container 5A which contains therein hydrofluoric acid A and a nitric acid container 5B which contains therein nitric acid B.
By heating the above-described hydrofluoric acid and nitric acid by means of heaters 7A and 7B, the closed top container is filled with highly-pure acid vapor composed of a hydrofluoric acid gas and a nitric acid gas until the container is saturated with the acid vapor. These gases are dissolved in pure water, whereby the sample kept in the sample container is dissolved.
It is however necessary to heat for 2 to 3 hours to saturate the closed top container with the acid vapor and then to dissolve the hydrofluoric acid and nitric acid in pure water. If the time necessary for dissolving the sample in the resulting solution is taken into consideration, 10-12 hours in total are necessary to dissolve 1 g of the silicon material. Thus, the above method requires long hours for dissolving the silicon material sample and is therefore not efficient.
In addition, if it takes much time to dissolve the silicon material sample, an amount of the contamination from surroundings shows an increase, which makes it impossible to carry out analysis with high sensitivity.